The invention relates to the manufacture of elastomeric-covered rollers.
Rollers with covers of elastomeric material are used in a wide variety of industrial applications. One process is a plastic film manufacturing process known as cast film extrusion. Films of thermoplastic polymers such as polyvinylchloride and polycarbonate are made by this process. The most popular polymers made by this process are polyethylene and polypropylene.
In a typical application, molten polymer from an extruder is dropped into a nip formed between a chrome-plated steel roller and an elastomer-covered steel roller. One common type of elastomer used in this application is silicone rubber, although Hypalon(trademark), available from Du Pont, and other polymer materials can also be used. The chrome-plated steel roller and the elastomer-covered roller are normally water cooled, or otherwise chilled, since the molten plastic may be at a temperature of several hundred degrees Fahrenheit. The chrome-plated roller quickly chills the plastic below its melting point and the nip formed with the elastomer-covered roller determines the gauge (thickness) and the surface finish of the film. The flexibility of the rubber covering compensates for minor machine misalignment and other variables, and provides a wider nip than two hard surfaced rollers.
Silicone rubber is a material that may be used as the cover in the elastomer-covered roller described for the above application. Silicone rubber is, however, a material that is difficult to bond to a metal roller core on a consistent basis. It is also difficult to maintain the bond in applications where the roller is used at high temperature and high pressure. In some cases, the silicone rubber may peel cleanly off the metal core without leaving any residue of rubber, indicating a loss of bonding. Loss of bonding in one area requires that the elastomer-covered roller be replaced.
The fact that the elastomer peels cleanly from the metal core surface, leaving little if any rubber residue on the core, indicates that the strength of the rubber/metal bond is weaker than that of the rubber itself. An adequate rubber/metal bond is generally considered to be one that is greater than the strength of the rubber. Bond interfaces that are initially adequate when formed can deteriorate relatively quickly under thermal or mechanical stresses in an actual field application. This is especially true for elastomers that are either inconsistent in their bonding properties or do not form particularly strong bonds to metals. Included in this group are silicone, EPDM (ethylene-propylene-diene-monomer), and urethane elastomers. Current methods of core surface preparation, prior to elastomeric bonding, include disc or belt sanding, shot blasting, sand blasting, and grit blasting. A cleanly tooled surface is not generally rough enough to promote a strong rubber to metal bond. These methods attempt to further clean the core surface of all oxides and contaminants, after solvents have been used to remove greases and oils, while increasing the surface area available for bonding. After the metal surface has been prepared, one or more chemical bonding agents are applied. These materials are specific to the type of elastomer being used and are well known in the industry. The total thickness of these bonding agents is frequently on the order of one mil.
The best surfaces for bonding that can be produced are by grit or shot blasting. The maximum roughness value that can be consistently achieved, with adequate process controls, is in the 500 microinches Ra range, but typical values for most processes are lower. Blasting increases the surface roughness and surface area but the surface profile is relatively simple, just peaks and valleys. The best grit blasted surface preparation will produce an adequate bond to the elastomer cover and will not peel cleanly at the rubber/metal bond interface under normal temperature and pressure stresses. However, the amount of rubber residue left on the core is relatively small.
A roller under mechanical stress, such as a silicone covered roller for example, has a concentration of stress at the rubber to metal interface, because of the vast difference in the compression modulus values between the elastomer and the core. This stress riser tends to shear the rubber away from the core at the bond interface.
An improved bonding surface is needed for elastomers that are weakly or inconsistently bonded, especially if the covered rollers are exposed to high temperature or pressures, or high moisture conditions. The bonding surface must provide a high surface area and surface roughness and yet be easily and consistently produced. A very high surface roughness will also diffuse the stress riser at the bond interface improving the longevity of the rubber to metal bond.
Polyurethane is another material that can be used for the elastomeric cover in the present invention. For bonding polyurethane to supporting layers, primary reliance has been placed on chemical bonding, to be assisted by mechanical bonding. As with silicone-based materials, the limit of surface roughness available with current methods of mechanical bonding is about 500 microinches Ra.
In addition, water vapor easily permeates through either a silicone rubber layer or a polyurethane layer, and corrosion may occur at the bond line due to the collection of water vapor there. Moisture will migrate into the outer roller cover if the roller is either chilled or exposed to water in the application. It would be beneficial to provide a bond coat or layer that is resistant to such corrosion, as well as one providing a stronger mechanical bond.
The invention concerns a method of making a roller that includes thermally spraying a bond coat to substantially cover a portion of the core on which an elastomeric layer is to be bonded and in which the bond coat provides a surface roughness to assure a strong mechanical bond to the elastomeric layer.
The invention also concerns a roller resulting from the method and having a bond coat with a surface roughness that provides a strong mechanical bond to an elastomeric layer.
The bond coat comprises a thermally sprayed material selected from a group of materials consisting of metals, metal alloys, ceramics and cermets.
In the prior art, core surfaces were prepared by sanding or blasting the metal core prior to the application of chemical bonding agents. These methods do not always produce the desired level of mechanical bonding. Thermal spraying has been known in the manufacture of ceramic covered rollers, but has not heretofore been applied to bond elastomeric layers to a metal core.
The invention provides improved constructions of elastomeric rollers. These elastomers include silicone, EPDM (ethylene-propylene-diene-monomer), urethane elastomers and other synthetic or natural rubber elastomers. Although the invention is described in examples in which the elastomeric layer is the outer layer, it would also be possible to add layers outside the elastomeric layer.
The invention may be practiced in further aspects by providing a two-layer bond coat in which the material of the base layer is denser for protection against corrosion and the top layer is coarser for a better bonding. Other objects and advantages of the invention, besides those discussed above, will be apparent to those of ordinary skill in the art from the description of the preferred embodiments which follow. In the description, reference is made to the accompanying drawings, which form a part hereof, and which illustrate examples of the invention. Such examples, however, are not exhaustive of the various embodiments of the invention, and therefore, reference is made to the claims which follow the description for determining the scope of the invention.